The proper functioning of the adult nervous system relies on the correct wiring of connections established during development. Defects in these processes can lead to devastating neurological disorders. The general goal of our research is to identify molecular and cellular mechanisms governing precise guidance events that achieve accurate wiring and patterning during development. Knowledge of the genes that regulate aspects of development such as axon guidance and the formation of neuro-vascular units will provide insights into the root causes of many human developmental neurological disorders. Increasing evidence shows that many disorders such as Alzheimer's disease, autism, and other neural degenerative and neural developmental disorders are caused by both neural and neuro-vascular defects. We have identified a novel ligand-receptor interaction between a vertebrate secreted semaphorin, Sema3E and its receptor Plexin-D1. We have also shown that Sema3E-Plexin-D1 is required in vivo for both nervous and vascular development. Sema3E also has been identified as a potential tumor angiogenesis inhibitor. Due to the relevance of this ligand/receptor pair to development of the nervous system and neural vascularization as well as its potential role in various neural developmental and degenerative disorders and as a cancer therapeutic target, we performed an image-based genome-wide RNAi screen to identify downstream genes required for Sema3E-Plexin-D1 signaling. The results of this screen identified several potential regulators for the Rho family small GTPase Rad as key downstream signaling molecules. These molecules will be validated by expression studies, in vitro biochemistry, and in vitro assays for axon guidance. These studies will elucidate the molecular mechanisms downstream of the novel ligand receptor pair, Sema3E/Plexin-D1, and their contributions to proper neural development. Understanding the cellular and molecular mechanisms of Sema3E/Plexin-D1 signaling will provide an understanding of how the nervous system develops and shed light on the mechanisms that lead to many neural developmental and neural degenerative diseases. In addition, this research is of particular interest because Sema3E is a known tumor-angiogenesis inhibitor and is a potential therapeutic target